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href="/noteslibrary/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/noteslibrary/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/noteslibrary/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/noteslibrary/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/noteslibrary/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/noteslibrary/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/noteslibrary/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/noteslibrary/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/noteslibrary/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/noteslibrary/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/noteslibrary/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/noteslibrary/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul></section></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/noteslibrary/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/noteslibrary/fontend/#前端" data-v-06225672>前端</a></li><li data-v-06225672><a href="/noteslibrary/fontend/#ES6笔记" data-v-06225672>ES6笔记</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">变量的解构赋值<!----></h1> <!----> <div class="theme-vdoing-content content__default"><h1 id="变量的解构赋值"><a href="#变量的解构赋值" class="header-anchor">#</a> 变量的解构赋值</h1> <h2 id="数组的解构赋值"><a href="#数组的解构赋值" class="header-anchor">#</a> 数组的解构赋值</h2> <h3 id="基本用法"><a href="#基本用法" class="header-anchor">#</a> 基本用法</h3> <p>ES6 允许按照一定模式，从数组和对象中提取值，对变量进行赋值，这被称为解构（Destructuring）。</p> <p>以前，为变量赋值，只能直接指定值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> c <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div> <p>ES6 允许写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> c<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码表示，可以从数组中提取值，按照对应位置，对变量赋值。</p> <p>本质上，这种写法属于“模式匹配”，只要等号两边的模式相同，左边的变量就会被赋予对应的值。下面是一些使用嵌套数组进行解构的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">[</span>bar<span class="token punctuation">]</span><span class="token punctuation">,</span> baz<span class="token punctuation">]</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
foo <span class="token comment">// 1</span>
bar <span class="token comment">// 2</span>
baz <span class="token comment">// 3</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span> <span class="token punctuation">,</span> <span class="token punctuation">,</span> third<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">&quot;foo&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;bar&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;baz&quot;</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
third <span class="token comment">// &quot;baz&quot;</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> <span class="token punctuation">,</span> y<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
x <span class="token comment">// 1</span>
y <span class="token comment">// 3</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>head<span class="token punctuation">,</span> <span class="token operator">...</span>tail<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
head <span class="token comment">// 1</span>
tail <span class="token comment">// [2, 3, 4]</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> <span class="token operator">...</span>z<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
x <span class="token comment">// &quot;a&quot;</span>
y <span class="token comment">// undefined</span>
z <span class="token comment">// []</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>如果解构不成功，变量的值就等于<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>bar<span class="token punctuation">,</span> foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>以上两种情况都属于解构不成功，<code>foo</code>的值都会等于<code>undefined</code>。</p> <p>另一种情况是不完全解构，即等号左边的模式，只匹配一部分的等号右边的数组。这种情况下，解构依然可以成功。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
x <span class="token comment">// 1</span>
y <span class="token comment">// 2</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>a<span class="token punctuation">,</span> <span class="token punctuation">[</span>b<span class="token punctuation">]</span><span class="token punctuation">,</span> d<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
a <span class="token comment">// 1</span>
b <span class="token comment">// 2</span>
d <span class="token comment">// 4</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面两个例子，都属于不完全解构，但是可以成功。</p> <p>如果等号的右边不是数组（或者严格地说，不是可遍历的结构，参见《Iterator》一章），那么将会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">NaN</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>foo<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面的语句都会报错，因为等号右边的值，要么转为对象以后不具备 Iterator 接口（前五个表达式），要么本身就不具备 Iterator 接口（最后一个表达式）。</p> <p>对于 Set 结构，也可以使用数组的解构赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Set</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">,</span> <span class="token string">'c'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
x <span class="token comment">// &quot;a&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>事实上，只要某种数据结构具有 Iterator 接口，都可以采用数组形式的解构赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">fibs</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
  <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> a<span class="token punctuation">;</span>
    <span class="token punctuation">[</span>a<span class="token punctuation">,</span> b<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span>b<span class="token punctuation">,</span> a <span class="token operator">+</span> b<span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> second<span class="token punctuation">,</span> third<span class="token punctuation">,</span> fourth<span class="token punctuation">,</span> fifth<span class="token punctuation">,</span> sixth<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">fibs</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
sixth <span class="token comment">// 5</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>fibs</code>是一个 Generator 函数（参见《Generator 函数》一章），原生具有 Iterator 接口。解构赋值会依次从这个接口获取值。</p> <h3 id="默认值"><a href="#默认值" class="header-anchor">#</a> 默认值</h3> <p>解构赋值允许指定默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>foo <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
foo <span class="token comment">// true</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token string">'b'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// x='a', y='b'</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token string">'b'</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// x='a', y='b'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>注意，ES6 内部使用严格相等运算符（<code>===</code>），判断一个位置是否有值。所以，只有当一个数组成员严格等于<code>undefined</code>，默认值才会生效。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token keyword">undefined</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
x <span class="token comment">// 1</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token keyword">null</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
x <span class="token comment">// null</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，如果一个数组成员是<code>null</code>，默认值就不会生效，因为<code>null</code>不严格等于<code>undefined</code>。</p> <p>如果默认值是一个表达式，那么这个表达式是惰性求值的，即只有在用到的时候，才会求值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'aaa'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，因为<code>x</code>能取到值，所以函数<code>f</code>根本不会执行。上面的代码其实等价于下面的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> x<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">===</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  x <span class="token operator">=</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
  x <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>默认值可以引用解构赋值的其他变量，但该变量必须已经声明。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">,</span> y <span class="token operator">=</span> x<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>     <span class="token comment">// x=1; y=1</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">,</span> y <span class="token operator">=</span> x<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>    <span class="token comment">// x=2; y=2</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">,</span> y <span class="token operator">=</span> x<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// x=1; y=2</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>x <span class="token operator">=</span> y<span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>     <span class="token comment">// ReferenceError: y is not defined</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面最后一个表达式之所以会报错，是因为<code>x</code>用<code>y</code>做默认值时，<code>y</code>还没有声明。</p> <h2 id="对象的解构赋值"><a href="#对象的解构赋值" class="header-anchor">#</a> 对象的解构赋值</h2> <h3 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h3> <p>解构不仅可以用于数组，还可以用于对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'aaa'</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'bbb'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
foo <span class="token comment">// &quot;aaa&quot;</span>
bar <span class="token comment">// &quot;bbb&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>对象的解构与数组有一个重要的不同。数组的元素是按次序排列的，变量的取值由它的位置决定；而对象的属性没有次序，变量必须与属性同名，才能取到正确的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> bar<span class="token punctuation">,</span> foo <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'aaa'</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'bbb'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
foo <span class="token comment">// &quot;aaa&quot;</span>
bar <span class="token comment">// &quot;bbb&quot;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span> baz <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'aaa'</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'bbb'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
baz <span class="token comment">// undefined</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码的第一个例子，等号左边的两个变量的次序，与等号右边两个同名属性的次序不一致，但是对取值完全没有影响。第二个例子的变量没有对应的同名属性，导致取不到值，最后等于<code>undefined</code>。</p> <p>如果解构失败，变量的值等于<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span>foo<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'baz'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
foo <span class="token comment">// undefined</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，等号右边的对象没有<code>foo</code>属性，所以变量<code>foo</code>取不到值，所以等于<code>undefined</code>。</p> <p>对象的解构赋值，可以很方便地将现有对象的方法，赋值到某个变量。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 例一</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span> log<span class="token punctuation">,</span> sin<span class="token punctuation">,</span> cos <span class="token punctuation">}</span> <span class="token operator">=</span> Math<span class="token punctuation">;</span>

<span class="token comment">// 例二</span>
<span class="token keyword">const</span> <span class="token punctuation">{</span> log <span class="token punctuation">}</span> <span class="token operator">=</span> console<span class="token punctuation">;</span>
<span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span> <span class="token comment">// hello</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码的例一将<code>Math</code>对象的对数、正弦、余弦三个方法，赋值到对应的变量上，使用起来就会方便很多。例二将<code>console.log</code>赋值到<code>log</code>变量。</p> <p>如果变量名与属性名不一致，必须写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> baz <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'aaa'</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'bbb'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
baz <span class="token comment">// &quot;aaa&quot;</span>

<span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">first</span><span class="token operator">:</span> <span class="token string">'hello'</span><span class="token punctuation">,</span> <span class="token literal-property property">last</span><span class="token operator">:</span> <span class="token string">'world'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">first</span><span class="token operator">:</span> f<span class="token punctuation">,</span> <span class="token literal-property property">last</span><span class="token operator">:</span> l <span class="token punctuation">}</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span>
f <span class="token comment">// 'hello'</span>
l <span class="token comment">// 'world'</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>这实际上说明，对象的解构赋值是下面形式的简写（参见《对象的扩展》一章）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> foo<span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> bar <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'aaa'</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'bbb'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>也就是说，对象的解构赋值的内部机制，是先找到同名属性，然后再赋给对应的变量。真正被赋值的是后者，而不是前者。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> baz <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'aaa'</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">'bbb'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
baz <span class="token comment">// &quot;aaa&quot;</span>
foo <span class="token comment">// error: foo is not defined</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>foo</code>是匹配的模式，<code>baz</code>才是变量。真正被赋值的是变量<code>baz</code>，而不是模式<code>foo</code>。</p> <p>与数组一样，解构也可以用于嵌套结构的对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token punctuation">[</span>
    <span class="token string">'Hello'</span><span class="token punctuation">,</span>
    <span class="token punctuation">{</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token string">'World'</span> <span class="token punctuation">}</span>
  <span class="token punctuation">]</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> <span class="token punctuation">{</span> y <span class="token punctuation">}</span><span class="token punctuation">]</span> <span class="token punctuation">}</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span>
x <span class="token comment">// &quot;Hello&quot;</span>
y <span class="token comment">// &quot;World&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>注意，这时<code>p</code>是模式，不是变量，因此不会被赋值。如果<code>p</code>也要作为变量赋值，可以写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token punctuation">[</span>
    <span class="token string">'Hello'</span><span class="token punctuation">,</span>
    <span class="token punctuation">{</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token string">'World'</span> <span class="token punctuation">}</span>
  <span class="token punctuation">]</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span> p<span class="token punctuation">,</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> <span class="token punctuation">{</span> y <span class="token punctuation">}</span><span class="token punctuation">]</span> <span class="token punctuation">}</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span>
x <span class="token comment">// &quot;Hello&quot;</span>
y <span class="token comment">// &quot;World&quot;</span>
p <span class="token comment">// [&quot;Hello&quot;, {y: &quot;World&quot;}]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>下面是另一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> node <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">loc</span><span class="token operator">:</span> <span class="token punctuation">{</span>
    <span class="token literal-property property">start</span><span class="token operator">:</span> <span class="token punctuation">{</span>
      <span class="token literal-property property">line</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
      <span class="token literal-property property">column</span><span class="token operator">:</span> <span class="token number">5</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span> loc<span class="token punctuation">,</span> <span class="token literal-property property">loc</span><span class="token operator">:</span> <span class="token punctuation">{</span> start <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token literal-property property">loc</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">start</span><span class="token operator">:</span> <span class="token punctuation">{</span> line <span class="token punctuation">}</span><span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token operator">=</span> node<span class="token punctuation">;</span>
line <span class="token comment">// 1</span>
loc  <span class="token comment">// Object {start: Object}</span>
start <span class="token comment">// Object {line: 1, column: 5}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码有三次解构赋值，分别是对<code>loc</code>、<code>start</code>、<code>line</code>三个属性的解构赋值。注意，最后一次对<code>line</code>属性的解构赋值之中，只有<code>line</code>是变量，<code>loc</code>和<code>start</code>都是模式，不是变量。</p> <p>下面是嵌套赋值的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> obj<span class="token punctuation">.</span>prop<span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">123</span><span class="token punctuation">,</span> <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

obj <span class="token comment">// {prop:123}</span>
arr <span class="token comment">// [true]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>如果解构模式是嵌套的对象，而且子对象所在的父属性不存在，那么将会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token punctuation">{</span>bar<span class="token punctuation">}</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">baz</span><span class="token operator">:</span> <span class="token string">'baz'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，等号左边对象的<code>foo</code>属性，对应一个子对象。该子对象的<code>bar</code>属性，解构时会报错。原因很简单，因为<code>foo</code>这时等于<code>undefined</code>，再取子属性就会报错。</p> <p>注意，对象的解构赋值可以取到继承的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> obj1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> obj2 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token string">'bar'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>obj1<span class="token punctuation">,</span> obj2<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token operator">=</span> obj1<span class="token punctuation">;</span>
foo <span class="token comment">// &quot;bar&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，对象<code>obj1</code>的原型对象是<code>obj2</code>。<code>foo</code>属性不是<code>obj1</code>自身的属性，而是继承自<code>obj2</code>的属性，解构赋值可以取到这个属性。</p> <h3 id="默认值-2"><a href="#默认值-2" class="header-anchor">#</a> 默认值</h3> <p>对象的解构也可以指定默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token punctuation">{</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
x <span class="token comment">// 3</span>

<span class="token keyword">var</span> <span class="token punctuation">{</span>x<span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
x <span class="token comment">// 1</span>
y <span class="token comment">// 5</span>

<span class="token keyword">var</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> y <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
y <span class="token comment">// 3</span>

<span class="token keyword">var</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> y <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">5</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
y <span class="token comment">// 5</span>

<span class="token keyword">var</span> <span class="token punctuation">{</span> <span class="token literal-property property">message</span><span class="token operator">:</span> msg <span class="token operator">=</span> <span class="token string">'Something went wrong'</span> <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
msg <span class="token comment">// &quot;Something went wrong&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>默认值生效的条件是，对象的属性值严格等于<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> <span class="token punctuation">{</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
x <span class="token comment">// 3</span>

<span class="token keyword">var</span> <span class="token punctuation">{</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token keyword">null</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
x <span class="token comment">// null</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，属性<code>x</code>等于<code>null</code>，因为<code>null</code>与<code>undefined</code>不严格相等，所以是个有效的赋值，导致默认值<code>3</code>不会生效。</p> <h3 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h3> <p>（1）如果要将一个已经声明的变量用于解构赋值，必须非常小心。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 错误的写法</span>
<span class="token keyword">let</span> x<span class="token punctuation">;</span>
<span class="token punctuation">{</span>x<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token comment">// SyntaxError: syntax error</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码的写法会报错，因为 JavaScript 引擎会将<code>{x}</code>理解成一个代码块，从而发生语法错误。只有不将大括号写在行首，避免 JavaScript 将其解释为代码块，才能解决这个问题。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 正确的写法</span>
<span class="token keyword">let</span> x<span class="token punctuation">;</span>
<span class="token punctuation">(</span><span class="token punctuation">{</span>x<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码将整个解构赋值语句，放在一个圆括号里面，就可以正确执行。关于圆括号与解构赋值的关系，参见下文。</p> <p>（2）解构赋值允许等号左边的模式之中，不放置任何变量名。因此，可以写出非常古怪的赋值表达式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token string">'abc'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面的表达式虽然毫无意义，但是语法是合法的，可以执行。</p> <p>（3）由于数组本质是特殊的对象，因此可以对数组进行对象属性的解构。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token number">0</span> <span class="token operator">:</span> first<span class="token punctuation">,</span> <span class="token punctuation">[</span>arr<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">]</span> <span class="token operator">:</span> last<span class="token punctuation">}</span> <span class="token operator">=</span> arr<span class="token punctuation">;</span>
first <span class="token comment">// 1</span>
last <span class="token comment">// 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码对数组进行对象解构。数组<code>arr</code>的<code>0</code>键对应的值是<code>1</code>，<code>[arr.length - 1]</code>就是<code>2</code>键，对应的值是<code>3</code>。方括号这种写法，属于“属性名表达式”（参见《对象的扩展》一章）。</p> <h2 id="字符串的解构赋值"><a href="#字符串的解构赋值" class="header-anchor">#</a> 字符串的解构赋值</h2> <p>字符串也可以解构赋值。这是因为此时，字符串被转换成了一个类似数组的对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token punctuation">[</span>a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> c<span class="token punctuation">,</span> d<span class="token punctuation">,</span> e<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
a <span class="token comment">// &quot;h&quot;</span>
b <span class="token comment">// &quot;e&quot;</span>
c <span class="token comment">// &quot;l&quot;</span>
d <span class="token comment">// &quot;l&quot;</span>
e <span class="token comment">// &quot;o&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>类似数组的对象都有一个<code>length</code>属性，因此还可以对这个属性解构赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token literal-property property">length</span> <span class="token operator">:</span> len<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
len <span class="token comment">// 5</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="数值和布尔值的解构赋值"><a href="#数值和布尔值的解构赋值" class="header-anchor">#</a> 数值和布尔值的解构赋值</h2> <p>解构赋值时，如果等号右边是数值和布尔值，则会先转为对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token literal-property property">toString</span><span class="token operator">:</span> s<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token number">123</span><span class="token punctuation">;</span>
s <span class="token operator">===</span> <span class="token class-name">Number</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>toString <span class="token comment">// true</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token literal-property property">toString</span><span class="token operator">:</span> s<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
s <span class="token operator">===</span> <span class="token class-name">Boolean</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>toString <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，数值和布尔值的包装对象都有<code>toString</code>属性，因此变量<code>s</code>都能取到值。</p> <p>解构赋值的规则是，只要等号右边的值不是对象或数组，就先将其转为对象。由于<code>undefined</code>和<code>null</code>无法转为对象，所以对它们进行解构赋值，都会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">prop</span><span class="token operator">:</span> x <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">undefined</span><span class="token punctuation">;</span> <span class="token comment">// TypeError</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">prop</span><span class="token operator">:</span> y <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// TypeError</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="函数参数的解构赋值"><a href="#函数参数的解构赋值" class="header-anchor">#</a> 函数参数的解构赋值</h2> <p>函数的参数也可以使用解构赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">]</span></span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">return</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">add</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，函数<code>add</code>的参数表面上是一个数组，但在传入参数的那一刻，数组参数就被解构成变量<code>x</code>和<code>y</code>。对于函数内部的代码来说，它们能感受到的参数就是<code>x</code>和<code>y</code>。</p> <p>下面是另一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span>a<span class="token punctuation">,</span> b<span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> a <span class="token operator">+</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// [ 3, 7 ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>函数参数的解构也可以使用默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">move</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>x <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span> y <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">8</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [3, 8]</span>
<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [3, 0]</span>
<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [0, 0]</span>
<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [0, 0]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，函数<code>move</code>的参数是一个对象，通过对这个对象进行解构，得到变量<code>x</code>和<code>y</code>的值。如果解构失败，<code>x</code>和<code>y</code>等于默认值。</p> <p>注意，下面的写法会得到不一样的结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">move</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>x<span class="token punctuation">,</span> y<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">0</span> <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">8</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [3, 8]</span>
<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [3, undefined]</span>
<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [undefined, undefined]</span>
<span class="token function">move</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [0, 0]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码是为函数<code>move</code>的参数指定默认值，而不是为变量<code>x</code>和<code>y</code>指定默认值，所以会得到与前一种写法不同的结果。</p> <p><code>undefined</code>就会触发函数参数的默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span>x <span class="token operator">=</span> <span class="token string">'yes'</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// [ 1, 'yes', 3 ]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="圆括号问题"><a href="#圆括号问题" class="header-anchor">#</a> 圆括号问题</h2> <p>解构赋值虽然很方便，但是解析起来并不容易。对于编译器来说，一个式子到底是模式，还是表达式，没有办法从一开始就知道，必须解析到（或解析不到）等号才能知道。</p> <p>由此带来的问题是，如果模式中出现圆括号怎么处理。ES6 的规则是，只要有可能导致解构的歧义，就不得使用圆括号。</p> <p>但是，这条规则实际上不那么容易辨别，处理起来相当麻烦。因此，建议只要有可能，就不要在模式中放置圆括号。</p> <h3 id="不能使用圆括号的情况"><a href="#不能使用圆括号的情况" class="header-anchor">#</a> 不能使用圆括号的情况</h3> <p>以下三种解构赋值不得使用圆括号。</p> <p>（1）变量声明语句</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 全部报错</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">x</span><span class="token operator">:</span> c<span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token punctuation">(</span>x<span class="token operator">:</span> c<span class="token punctuation">)</span><span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token operator">:</span> c<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span> <span class="token literal-property property">o</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> p <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">o</span><span class="token operator">:</span> <span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token number">2</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面 6 个语句都会报错，因为它们都是变量声明语句，模式不能使用圆括号。</p> <p>（2）函数参数</p> <p>函数参数也属于变量声明，因此不能带有圆括号。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
<span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span><span class="token punctuation">(</span>z<span class="token punctuation">)</span><span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> z<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token comment">// 报错</span>
<span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span>z<span class="token punctuation">,</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> x<span class="token punctuation">;</span> <span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>（3）赋值语句的模式</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 全部报错</span>
<span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> a <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token number">42</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">(</span><span class="token punctuation">[</span>a<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码将整个模式放在圆括号之中，导致报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
<span class="token punctuation">[</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> a <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token punctuation">{</span> <span class="token literal-property property">x</span><span class="token operator">:</span> c <span class="token punctuation">}</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码将一部分模式放在圆括号之中，导致报错。</p> <h3 id="可以使用圆括号的情况"><a href="#可以使用圆括号的情况" class="header-anchor">#</a> 可以使用圆括号的情况</h3> <p>可以使用圆括号的情况只有一种：赋值语句的非模式部分，可以使用圆括号。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">[</span><span class="token punctuation">(</span>b<span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// 正确</span>
<span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">p</span><span class="token operator">:</span> <span class="token punctuation">(</span>d<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 正确</span>
<span class="token punctuation">[</span><span class="token punctuation">(</span>parseInt<span class="token punctuation">.</span>prop<span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// 正确</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面三行语句都可以正确执行，因为首先它们都是赋值语句，而不是声明语句；其次它们的圆括号都不属于模式的一部分。第一行语句中，模式是取数组的第一个成员，跟圆括号无关；第二行语句中，模式是<code>p</code>，而不是<code>d</code>；第三行语句与第一行语句的性质一致。</p> <h2 id="用途"><a href="#用途" class="header-anchor">#</a> 用途</h2> <p>变量的解构赋值用途很多。</p> <p><strong>（1）交换变量的值</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> y <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span>y<span class="token punctuation">,</span> x<span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码交换变量<code>x</code>和<code>y</code>的值，这样的写法不仅简洁，而且易读，语义非常清晰。</p> <p><strong>（2）从函数返回多个值</strong></p> <p>函数只能返回一个值，如果要返回多个值，只能将它们放在数组或对象里返回。有了解构赋值，取出这些值就非常方便。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 返回一个数组</span>

<span class="token keyword">function</span> <span class="token function">example</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> <span class="token punctuation">[</span>a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> c<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">example</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 返回一个对象</span>

<span class="token keyword">function</span> <span class="token function">example</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token literal-property property">foo</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
    <span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token number">2</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token function">example</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p><strong>（3）函数参数的定义</strong></p> <p>解构赋值可以方便地将一组参数与变量名对应起来。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 参数是一组有次序的值</span>
<span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span>x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z<span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 参数是一组无次序的值</span>
<span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>x<span class="token punctuation">,</span> y<span class="token punctuation">,</span> z<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">z</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token literal-property property">y</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token literal-property property">x</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p><strong>（4）提取 JSON 数据</strong></p> <p>解构赋值对提取 JSON 对象中的数据，尤其有用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> jsonData <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token literal-property property">id</span><span class="token operator">:</span> <span class="token number">42</span><span class="token punctuation">,</span>
  <span class="token literal-property property">status</span><span class="token operator">:</span> <span class="token string">&quot;OK&quot;</span><span class="token punctuation">,</span>
  <span class="token literal-property property">data</span><span class="token operator">:</span> <span class="token punctuation">[</span><span class="token number">867</span><span class="token punctuation">,</span> <span class="token number">5309</span><span class="token punctuation">]</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> <span class="token punctuation">{</span> id<span class="token punctuation">,</span> status<span class="token punctuation">,</span> <span class="token literal-property property">data</span><span class="token operator">:</span> number <span class="token punctuation">}</span> <span class="token operator">=</span> jsonData<span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>id<span class="token punctuation">,</span> status<span class="token punctuation">,</span> number<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 42, &quot;OK&quot;, [867, 5309]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码可以快速提取 JSON 数据的值。</p> <p><strong>（5）函数参数的默认值</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>jQuery<span class="token punctuation">.</span><span class="token function-variable function">ajax</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span>url<span class="token punctuation">,</span> <span class="token punctuation">{</span>
  async <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
  <span class="token function-variable function">beforeSend</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
  cache <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
  <span class="token function-variable function">complete</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
  crossDomain <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
  global <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
  <span class="token comment">// ... more config</span>
<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ... do stuff</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>指定参数的默认值，就避免了在函数体内部再写<code>var foo = config.foo || 'default foo';</code>这样的语句。</p> <p><strong>（6）遍历 Map 结构</strong></p> <p>任何部署了 Iterator 接口的对象，都可以用<code>for...of</code>循环遍历。Map 结构原生支持 Iterator 接口，配合变量的解构赋值，获取键名和键值就非常方便。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
map<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">'first'</span><span class="token punctuation">,</span> <span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
map<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token string">'second'</span><span class="token punctuation">,</span> <span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">,</span> value<span class="token punctuation">]</span> <span class="token keyword">of</span> map<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key <span class="token operator">+</span> <span class="token string">&quot; is &quot;</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// first is hello</span>
<span class="token comment">// second is world</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>如果只想获取键名，或者只想获取键值，可以写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 获取键名</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token keyword">of</span> map<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>

<span class="token comment">// 获取键值</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> <span class="token punctuation">[</span><span class="token punctuation">,</span>value<span class="token punctuation">]</span> <span class="token keyword">of</span> map<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p><strong>（7）输入模块的指定方法</strong></p> <p>加载模块时，往往需要指定输入哪些方法。解构赋值使得输入语句非常清晰。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> <span class="token punctuation">{</span> SourceMapConsumer<span class="token punctuation">,</span> SourceNode <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">&quot;source-map&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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